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when he could/would have done sth

He paid for a seat, when he ______ have entered free.
A. would B. must C. could D. need

The question above is taken from one of China's province's 2005 college entrance examination paper. The given answer is C. I'd like to know if we can also use A (would) for the answer. And could we simply say 'He paid for a seat, when he could enter free'?